Xanthan gum is produced by bacteria of the genus Xanthomonas, such as the species campestris, albilineans, fragaria, vesicatoria, and the like. Xanthan gum is a widely used product due to its unusual physical properties: extremely high specific viscosity and pseudoplasticity. It is commonly used in foods as a thickening agent and in secondary oil recovery as mobility control and profile modification agents and in petroleum drilling fluids.
Chemically, xanthan gum is an anionic heteropolysaccharide. The repeating unit of the polymer is a pentamer composed of five sugar moieties: two glucose, one glucuronic acid and two mannose moieties. They are arranged such that the glucose moieties form the backbone of the polymer chain, and side chains of mannose-glucuronic acid-mannose generally extend from alternate glucose moieties. Often this basic structure is specifically acetylated and/or pyruvylated. (Janson, P. E., Kenne, L., and Lindberg, B., Carbohydrate Research, 45, 275-282 (1975); Melton, L. D., Mindt, L., Rees, D. A., and Sanderson, G. R., Carbohydrate Research, 46, 245-257 (1976).) This and all other publications referred to herein are specifically incorporated by reference. The structure is depicted below: ##STR1##
In spite of the broad utility of naturally occurring xanthan gum, there are some situations where its physical properties become limiting. In particular, in secondary oil recovery it is not uncommon for the temperature of the oil-bearing reservoir and salt concentrations in the reservoir brine to be higher than are optimal for xanthan solutions. When these conditions occur, xanthan can precipitate, flocculate and/or lose viscosity. Therefore there is a need for new viscosifying products which perform well at high temperature and high salt conditions.